High reliability is generally required for laser diodes, such as pump lasers and signal lasers, used in fiber-optic communication. Packaging these lasers typically involves the following steps: fixing the laser and other parts in an enclosure; having optics to couple the light from the laser chip to an optical fiber; and sealing the enclosure hermetically. Pump laser modules, which nowadays have hundreds of mW fiber-coupled optical power, are the key components in erbium doped fiber amplifiers. Because of the high power output, packaging pump laser modules is more challenging in both the package design and process as compared to those for the low power lasers.
One of the major challenges in packaging of high power output fiberoptic components is the tight tolerance in fiber alignment (FIG. 1). The laser beam emitted from the laser chip 10 (FIG. 2) is elliptical with an aspect ratio of about 3:1. In order to maximize the power coupling from the chip to the single-mode optical fiber 12, a wedge-shape lensed fiber is commonly used, the end 14 of which acts as a micro cylindrical lens to convert the elliptical beam into a circular one. Coupling efficiency of 70% or higher is readily achievable by using lensed fibers. Both optical modeling and actual measurement indicate (as shown in FIG. 1) that to stay within 90% of the peak coupling efficiency, the translation tolerance (x and z-axes, see FIG. 2) of fiber to chip alignment is on the order of micrometer; and in the vertical direction (y-axis) the tolerance is even reduced to sub-micrometer.
In addition to the tight tolerance, it is also required that the high-power fiber optic package is free from any contamination, including outgassing, of organic materials. Such a contamination could be laser dissociated into carbon particles, which then deposit onto the laser facet. The contamination will, therefore, degrade the laser performance and could eventually cause catastrophic optical damage to the laser chip.
Laser welding has been used in packaging pump laser modules for many years and has been proven as a reliable and clean process. In this process, the lensed fiber is assembled in a metal ferrule and a special designed metal clip is needed for welding the fiber to a metal base. Because the laser welding process inevitably introduces some local stress on the welding spot and hence, shifts the fiber out of alignment, one has to anneal the laser module to relax the excessive stress and then bring the fiber back into alignment by “hammering” which means to physically deform the metal parts. The annealing/hammering process could take several iterations before the fiber is stabilized in alignment.
Low temperature solder glasses (also known as frits) are proven materials and are widely used in opto-electronic packages as hermetic sealing compounds for feed-through pins, optical windows and lenses. Solder glass starts in a form of powder or preforms and is processed at high temperature that ranges from 350° C. to 700° C., depending on the specific composition. It is non-creeping, clean and stable over the temperature range that laser modules are specified.